The velocity structure of the continental margin off Van Mijenfjorden can be used for a simple schematic Cenozoic evolution model, since it reveals information about the West Spitsbergen Orogeny as well as the rifting procedure in Oligocene.
The Spitsbergen Orogeny was a Late Paleocene transpressive event (Steel et al., 1985;
Eldholm et al., 1987). According to our observations along the seismic refraction profile we characterise the deeper roots of the fold belt as intensively faulted rocks, that are most probably related to oblique convergent wrenching. This gave way to transtension since the Oligocene (Eldholm et al., 1987) and the intensively faulted rock units entered a relaxing phase (Fig. 4-11a). Crustal thinning occurred but is limited to the Caledonian western terrane of Svalbard. These observations are similar to those made along the seis- mic refraction transect off Kongsfjorden (Ritzmann et al., subm to Marine Geophys.
THE DEVELOPMENT OF THE CONTINENTAL MARGIN OFF VAN MIJENFJORDEN
I
'03Greenland Svalbard
1
V Transtensional Riftingl N'Migration of KR
Gre Shel
Greenland Svalbard
from KR
4- +
MTF?
Seafloor Spreading
Figure 4-11: Schematic evolution of the continental margin o f f Van Mijenfjorden derived from the Interpretation of seismic refraction profiles AWI-97260 and profile 9 .
Diiritig the Paleocene a transpressional reginze dominated behveeti Svalbard und Greenland arid resulted in the West Spitsbergen Fold Belt (cross hatch pattern). Sincejirst subsidence of the For- landsundet Graben is achieved dwing this phase we propose a si~nilar history for the Bellsund Graben (BG) o f f Van Mijenfjorden (VM). Intensive shearing iveakened the upper brittle part of the crust. The vertical dotted iine indicates approxitnately the boundary behveen the central arid the western terrane. ( U ) Since Oligocene transte~zsion occzirred and the Hornsund Li~zeamerit (HL) was the active fault System between Svalbard und Greenland. Stress was taken off the Fold Belt (Bellsutid region), and thinning of the western terrane occzirred decoupledfrom central Svalbard.
West of the HL block faulting tookplace. Note that the upper ernst is supposed to comprise Paleo- zoic to Mesozoic consolidated sedin~entwy rocks (grey box). The northern segment of the Knipov- ich Ridge (KR) was situated directly adjacent to the slightly thinned crmt off Van Mijetfjorden.
Mantle det-ived meltsfrom the deeper Knipovich Ridge were injected into the lower ernst at the central I-ift. Later (b) the mid-oceati ridge System migrated further north. Rifted crustal sections were tilted and buried below younger sediments.
Res., See chapter 3). Both profiles reveal that crustal thinning is decoupled from inner Svalbard (the central terrane), probably the consequence of the former transpressive suture of the West Spitsbergen Fold Belt, that itself is located in close proximity to the Caledonian suture.
104 CRUSTAL STRUCTURE BETWEEN THE KNIPOVICH RIDGE AND THE VAN MIJENFJORDEN (SVALBARD)
The main fault Zone between Svalbard and Greenland was most probably the Hom- sund Lineament (Eiken, 1993) that is supposed to bound a Paleozoic (Carboniferous?) os Mesozoic sedimentary basin to the east (e.g. Mann & Townsend, 1989; Townsend &
Mann, 1989). The Knipovich Ridge is supposed to have been located adjacent to Van Mijenfjorden during the Miocene (Crane et al., 1991; Boebel, 2000). Elevated lower crustal velocities are found at the continent-ocean transition and we suppose magmatic interaction between the deeper mid-oceanic ridge and the continental crust, keeping in mind that a rifted-volcanic evolution can be excluded (Ritzmann & Jokat, 2003; see chapter 2). As the mid-ocean ridge System migrated further north of the Knipovich Ridge (Fig. 4-1 lb) opening of the Fram Strait, continental crust off Van Mijenfjorden was fur- ther thinned, rifted and probably block-faulted (Eiken & Austegard, 1987). The Horn- sund Lineament developed. It remains unclear if the Molloy Transform Fault developed prior to the northern Knipovich Ridge leading to a sheared margin in the outer Zone of the continent-ocean transition (km 1 10).
4.7 Conclusions
The main results of our study are:
(1) Cenozoic sediments and sedimentary rocks On- and offshore exhibit seismic velocities from 4.5 to 5.4 km/s (Spitsbergen Tertiary Basin, onshore) and 1.7 and 3.9 km/s (offshore) with maximum thicknesses of 3.5 and 5 km, respectively. Below the Cenozoic cover onshore are up to 8 km of Devonian rocks associated with the Nordfjorden Block. West of the Hornsund Lineament the velocity structure gives rise to the suspicion that Paleozoic (Carboniferous) sedimentary Strata occur below the Cenozoic cover.
(2) The western and central Caledonian terranes can be differentiated, the boundary occurs at the western termination of the Devonian deposits on the Nordfjorden Block. Seismic velocities range from 5.4 km/s at the top (W'AkselGya) to 6.8 km/s at the Moho. The upper and middle crust at the West Spitsbergen Fold Belt reveals decreased seismic velocities that are interpreted as an intensively faulted rock construction due to transpressive movements dur- ing the West Spitsbergen Orogeny. The lower parts of the rifted ernst show a small Zone of slightly elevated velocities (7.2 km/s). These are inteipreted as mantle derived mafic or ultramafic rocks, intmded while the northem Knipo- vich Ridge was adjacent to the young rift off Van Mijenfjorden.
(3) The oceanic crust related to the Knipovich Ridge shows maximum thick- nesses of 4.0 km. The seismic velocities observed within oceanic crust indicate the absence of layer 3 (3.514.1 to 4.7 km/s). Minimum thicknesses of 1 to 2 km are achieved in narrow zones, interpreted as fracture zones. Below these frac- tures the seismic velocity of the upper mantle is low (7.3 krn/s) leading to the assumption that the mantle is partly serpentinised. The distribution of fracture zones and thicker crustal sections (30 to 50 kmlspacing) give new constraints On the segmentation of the northern Knipovich Ridge.
(4) Density modelling along the seismic refraction transect confirms the observed velocity structure for the continental and transitional crustal sections.
We assume an extreme mismatch of up to 150 mGal along the oceanic profile section is due to a more complex density (i.e. thermal) stmcture of the upper mantle.
The deep stmcture formed during the Cenozoic evolution of the West Spitsber- gen Fold Belt off Van Mijenfjorden are similar to those observed off Kongs- fjorden (Ritzmann et al., subm. to Marine Geophys. Res., see chapter 3). But the evolution of the continental margin differs since a rifted margin (instead of sheared) is observed off Van Mijenfjorden. Both margins reveal probable con- tamination by mantle derived ultramafic melts resulting in slightly elevated seismic velocities. In both cases the close proximity of the young plate bound- ary to the continental cmst is considered to be responsible,
AIM OF THIS CHAPTER 109
CHAPTER 5:
ADDITIONAL SEISMIC REFRACTION DATA ACQUIRED IN
1997: PROFILE AWI-99200
Figure 5-1: Location of seismic refraction projile AWI-99200.
The projile is marked by a tliick black liiie, RejTek- uiid OBS- und OBH sta- tions by black, grey und white triaiigles, respectively. Geology: Harlaiid ( 1 9 9 7 ~ ) . Black liiles are major faults onshore und offshore, nuinbered circles mark the following structures: ( I ) Molloy Transform Fault, ( 2 ) Raudfjorden Fault Zone, (3) Breibogen Fault Zone, (4) Billefjorden Fault Zone. Leiters indicate the follo~~~iiig locations: ( U ) Sjiibrebankeiz, ( b ) Kortgsf]ordei~IForlai~d- siuidet, ( C ) Mo1lo.y Deep ( d ) Danskftya Basiii, ( e ) Hinlopen Streetet, (f) Wijde- fjorden, ( g ) Woodfjorden. Bathymeti-,~: Jakobsson et 01. (2000).
5.1 Aim of this chapter
The seismic refraction profiles discussed in the chapters 2 to 4 were gathered during expeditions ARK13/2 and ARK15/2 of RV Polarstern. During the latter expedition addi- tional seismic refraction data were gathered by the Alfred Wegener Institute on a profile along the northern coast of Svalbard extending from Western Nordaustlandet to the Mol- loy Deep (Jokat et al., 2000; Fig. 5-1). In order to complete the presentation of the deep
110 ADDITIONAL SEISMIC REFRACTION DATA ACQUIRED IN 1997: PROFILE AWI-99200
l
seismic data off Svalbard this chapter deals with profile AWI-99200. The profile com- pletes a sequence of W-E striking seismic refraction profiles across Svalbard's western continental margin. Thus, a dense pattem of new seismic lines constrains local differ- ences along the margin and provides spatial information about margin segmentation that was mentioned earlier in this thesis (section 3.6 (5)).
Profile AWI-99200 was first modelled by Czuba et al. (in prep). The following discus- sion of the profile is a second, independent inte~pretation.